Question 115471 in Mongodb Question posted in
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Mongodb aggregation convert array of pairs to key and list of values

Trying to condense an array with key value pairs into an array of objects with the key and all the unique values for that key.

I have a structure like:

{
  fruits: [
    {fruit: apple, type: gaja},
    {fruit: apple, type: honey-crisp},
    {fruit: apple, type: fuji},
    {fruit: cherry, type: black},
    {fruit: cherry, type: red},
    {fruit: cherry, type: red},
  ]
}

How can I convert it to:

{
  fruits: [
    {fruit: apple, types: [gaja, honey-crisp, fuji]},
    {fruit: cherry, types: [black, red]}
  ]
}

Using mongo aggregations I managed to get the first structure from my data using $group and $addToSet. Not sure how to map the array to new object with a key and list of values

Tags:

3

Answers


  1. 0

    Maybe something like this:

    db.collection.aggregate([
{
  $unwind: "$fruits"
},
{
 $group: {
  _id: "$fruits.fruit",
  type: {
    $push: "$fruits.type"
  }
 }
},
{
 $project: {
  fruit: "$_id",
  type: 1,
  _id: 0
 }
},
{
$group: {
  _id: "",
  fruits: {
    $push: "$$ROOT"
  }
 }
 }
])

    Explained:

    1. Unwind the array
    2. Group to form the type array ( you can use $push or $addToSet in case you need only unique )
    3. Project the necessary fields
    4. Group all documents inside single final one

    Playground

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  2. 0

    Here’s another way to do it by using "$reduce". Comments are in the aggregation pipeline.

    db.collection.aggregate([
  {
    "$set": {
      // rewrite fruits
      "fruits": {
        "$reduce": {
          "input": "$fruits",
          "initialValue": [],
          "in": {
            "$let": {
              "vars": {
                // get fruit index in $$value : will be -1 if not there
                "idx": {"$indexOfArray": ["$$value.fruit", "$$this.fruit"]}
              },
              "in": {
                "$cond": [
                  // is fruit not in $$value yet
                  {"$eq": ["$$idx", -1]},
                  // new fruit so put in $$value and make "type" an array
                  {
                    "$concatArrays": [
                      "$$value",
                      [{"$mergeObjects": ["$$this", {"type": ["$$this.type"]}]}]
                    ]
                  },
                  // fruit already in $$value, so map $$value with "type" update
                  {
                    "$map": {
                      "input": "$$value",
                      "as": "val",
                      "in": {
                        "$cond": [
                          // is this array element not the right fruit?
                          {"$ne": ["$$val.fruit", "$$this.fruit"]},
                          // nope, leave the element as-is
                          "$$val",
                          // this element needs to be updated
                          {
                            "$mergeObjects": [
                              "$$val",
                              {
                                "type": {
                                  "$cond": [
                                    // is this "type" already in array?
                                    {"$in": ["$$this.type", "$$val.type"]},
                                    // yes, so leave it as-is
                                    "$$val.type",
                                    // this is a new "type", so add it to array
                                    {"$concatArrays": ["$$val.type", ["$$this.type"]]}
                                  ]
                                }
                              }
                            ]
                          }
                        ]
                      }
                    }
                  }
                ]
              }
            }
          }
        }
      }
    }
  }
])

    Try it on mongoplayground.net.

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  3. 0

    Here’s another, another way using a multiple "$map" and "$setUnion" to get unique array members.

    db.collection.aggregate([
  {
    "$set": {
      // rewrite fruits
      "fruits": {
        "$map": {
          // map over unique fruits
          "input": {"$setUnion": "$fruits.fruit"},
          "as": "theFruit",
          "in": {
            // set fruit
            "fruit": "$$theFruit",
            // "type" are unique elements of fruits.type
            // where fruits.fruit == theFruit
            "type": {
              "$setUnion": {
                "$map": {
                  "input": {
                    "$filter": {
                      "input": "$fruits",
                      "as": "obj",
                      "cond": {"$eq": ["$$obj.fruit", "$$theFruit"]}
                    }
                  },
                  "in": "$$this.type"
                }
              }
            }
          }
        }
      }
    }
  }
])

    Try it on mongoplayground.net.

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